A considerable amount of food is preserved using sealed metal cans. Many of such cans will be sealed by joining the lid and the body of the can by a "double seam" which forms a rim or flange circumferentially around the top of the can. Typically, after the can has been filled, the sealing operation is performed in a partial vacuum by rolling the metal of the lid and the body tightly together with a small amount of sealing compound.
While the technology for sealing cans is well developed, a variety of flaws or faults (herein referred to as "irregularities") in the resulting seam can occur; for example: cracks, knocked down rims, false seams, droop, and the like. Since the integrity of the seam can be crucial to the avoidance of spoilage and preservation of food within the can, it becomes highly desirable to provide a means of inspecting cans against the occurrence of such irregularities.
Historically, canners have sought to assure seam integrity by a variety of means, the most basic being manual inspection by the human eye. Of course, the manual approach may only suffice for relatively gross defects that can be easily seen by an inspector, and then only so long as he or she pays constant attention to the matter at hand. Furthermore, the manual approach can be compromised by high speed canning lines where it is impractical to manually inspect every can. To circumvent this limitation, methods have been developed to manually accept or reject batches of cans on the basis of statistical sampling techniques.
Automatic equipment used for the purpose of can inspection is frequently based on the assumption that a properly sealed can will have a partial vacuum. If a seal is faulty, then air leakage will reduce deflection of the can lid. Thus, so called "dud detectors" measure can end deflection, rejecting those cans where the measured deflection is not within prescribed limits. The prior art reveals a variation in which a can is struck electromagnetically, and the vibrations induced in the can used as an indicator of proper pressure.
There are significant problems with this general approach. Measurements of can deflection can be influenced by a variety of factors, including metal thickness of the can ends, the strength of the vacuum applied during canning, the amount and firmness of the fill, and the countersink depth as a result of settings for the seam rollers on the canning machine. In addition, a flaw has to be relatively serious for leakage to occur during the typically brief time that a can spends on the processing line. Many types of flaws may not lead to significant leakage until later. Research has shown that even relatively small problems in the seam may lead to leakage and that multiple minor impacts can be as significant as one major blow.